nfp: tls: fix error return code in nfp_net_tls_add()

Fix to return negative error code -EINVAL from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 1f35a56c ("nfp: tls: add/delete TLS TX connections")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>